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Abstract

Real-time display of processed en-face spectral domain optical coherence tomography (SD-OCT) images is important for diagnosis. However, due to many steps of data processing requirements, such as Fast Fourier transformation (FFT), data re-sampling, spectral shaping, apodization, zero padding, followed by software cut of the 3D volume acquired to produce an en-face slice, conventional high-speed SD-OCT cannot render an en-face OCT image in real time. Recently we demonstrated a Master/Slave (MS)-OCT method that is highly parallelizable, as it provides reflectivity values of points at depth within an A-scan in parallel. This allows direct production of en-face images. In addition, the MS-OCT method does not require data linearization, which further simplifies the processing. The computation in our previous paper was however time consuming. In this paper we present an optimized algorithm that can be used to provide en-face MS-OCT images much quicker. Using such an algorithm we demonstrate around 10 times faster production of sets of en-face OCT images than previously obtained as well as simultaneous real-time display of up to 4 en-face OCT images of 200 × 200 pixels2 from the fovea and the optic nerve of a volunteer. We also demonstrate 3D and B-scan OCT images obtained from sets of MS-OCT C-scans, i.e. with no FFT and no intermediate step of generation of A-scans.

Fig. 2 Processing block showing the procedure of extracting the reflectivity Ap for a single point at depth zp using (a): the traditional FFT based SD method and (b): the correlation based MS interferometry method.

Fig. 3 Time required to infer the reflectivity of a point at depth, t1 (shown on the left vertical axis) using the short correlation method versus the window size, W (circles). The red horizontal lines indicate the time (values shown on the right vertical axis) required by the conventional FFT-based SD-OCT as well as the time of two other MS-OCT methods, evaluated for W = 255. The values on the vertical axis on the right are obtained by simple multiplication of the left vertical axis values by L2 = 40000.

Fig. 5 36 en-face images of the fovea area obtained using the short correlation method (W = 10) showing different retinal and choroidal layers. Each image is 200 × 200 pixels. The depth separation between consecutive images is 25 µm measured in air, as dictated by the masks acquired in the Preparation stage. The eye was placed in such a way to avoid mirror terms perturbing the images. Lateral image size: 3 mm × 3 mm. The first image (top, left) is recorded at an approximate depth z = 100 µm.

Fig. 6 48 en-face images of the optic nerve area obtained using the short correlation method (using W = 10) showing the lamina cribrosa. Each image is 200 × 200 pixels. The depth separation between consecutive images is 25 µm measured in air, as determined by the set of masks acquired in the Preparation stage. The eye orientation and voltage applied to the galvo-scanners were adjusted to fit all the lamina in the middle of the image. The optic nerve was placed slightly away from OPD = 0 to avoid mirror terms perturbing the images. The first image (top, left) is recorded at a depth z = 50 µm.

Fig. 8 Top (a-d): four single-frame excerpts from a movie (Media 1) recorded in real-time showing C-scan MS-OCT images of the fovea area obtained using the short correlation method (W = 10). Each image is 200 × 200 pixels. The depth separation between images is 25 µm measured in air. Bottom (e): single frame excerpt from the same movie showing not resampled B-scans OCT images used for eye guidance at 250 Hz. All C-scan and B-scan images are displayed simultaneously. The first image (top, left) is recorded at a depth z = 50 µm.

Fig. 9 Top (a-d): four single-frame excerpts from a movie (Media 2) recorded in real-time showing C-scan MS-OCT images of the optic nerve area obtained using the short correlation method (W = 10). Each image is 200 × 200 pixels. The depth separation between images is 50 µm measured in air. Bottom (e): single frame excerpt from the same movie showing the non resampled B-scans OCT images used for eye guidance. All C-scan and B-scan images are displayed simultaneously. The first image (top, left) is recorded at a depth z = 50 µm.